This application is related to Japanese Patent Application No. HEI 11-146141 filed on May 26, 1999, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a thermomagnetic recording medium, a thermomagnetic recording and reproducing method and a thermomagnetic recording and reproducing device for recording information with use of heat and magnetism and reproducing optical or magnetic signals with use of heat.
2. Description of Related Art
Recently, remarkable development has been seen in technology for high-density optical memories as typified by DVDs (digital versatile disks) and magneto-optical disks, and a recording density over several Gbits/in2 has already been realized. However, higher density is still desired for mass-storage devices for so-called home servers intended to store motion video pictures.
The recording density of such optical memories is basically determined by the diameter of a light spot formed by irradiation of laser light or other kind of light (more precisely, temperature distribution formed by the light spot). Therefore, now proceeding is the development of a purplish blue laser for providing a light source of a shorter wavelength and the increase of numerical aperture (NA) by use of a solid immersion lens (SIL), with the intention of reducing the light spot diameter (defined as a range within which light intensity does not fall below 1/e2).
In addition to such techniques of reducing the light spot diameter, the recording density of magneto-optical memories is being raised by a magnetic superresolution technique by skillful use of the temperature distribution formed by the light spot and temperature characteristics of a magnetic medium.
Besides magneto-optical memories, proposed are new high-density magnetic memories in which optical technology and magnetic technology are combined. For example, Japanese Patent Publication No. 2617025 discloses recording on and reproduction from a narrow track corresponding to the light spot diameter with use of a wide magnetic head without cross talk by xe2x80x9cemploying a ferrimagnetic substance having a compensation point at room temperature and applying a light beam at recording and reproducing for raising temperaturexe2x80x9d (the disclosed memory will be referred to as an optically assisted magnetic memory hereinafter).
To control the temperature distribution of media is considered to be an effective way of improving the recording density of memories such as the above-described magneto-optical memories and optically assisted memory which employ both heat and magnetism for recording and reproduction (referred to as thermomagnetic recording hereinafter).
For controlling the temperature distribution in such media, there are seen a lot of techniques of designing the thermal conductivity itself of layers of media with varying materials therefor, and proposed is the use of xe2x80x9ca layer whose thermal conductivity changes with temperature (referred to as a thermal change layer).xe2x80x9d
For example, Japanese Unexamined Patent Publication No. HEI 2(1990)-304750 discloses the provision for a magneto-optical medium of a reflection film xe2x80x9cwhose thermal conductivity drops at around a magnetic transition temperaturexe2x80x9d for preventing a decline in a carrier to noise (C/N) ratio due to thermal interference between adjacent bits and also discloses the use of magnetic phase transition (Nxc3xa9el temperature=170xc2x0 C.) of a Cr film, which is an anti-ferromagnetic substance, for this reflection film which acts as a thermal change layer.
Also, for an example of a magneto-optical medium, Japanese Unexamined Patent Publication No. HEI 3(1991)-209649 discloses xe2x80x9cthe control of changes in temperature gradient in a region irradiated by laser lightxe2x80x9d by use of xe2x80x9ca recording assist film comprised of a material whose thermal conductivity declines with rise of temperaturexe2x80x9d and also discloses the use of a TiN film, a ZrN film and a SnO2 film as this recording assist film which acts as a thermal change layer.
Further, Japanese Unexamined Patent Publication No. HEI 10(1998)-40580 discloses an example of a phase change optical medium in which a thermal change layer xe2x80x9cwhose thermal conductivity changes with increase of light intensityxe2x80x9d is used for xe2x80x9cdecreasing the size of recording marks while maintaining an operational power marginxe2x80x9d with the intention of obtaining higher density and also discloses the use of a xe2x80x9clow-melting metalxe2x80x9d for this thermal change layer.
However, it has been found that the above-described conventional techniques have problems in the principles themselves of realizing the xe2x80x9cchange of the thermal conductivityxe2x80x9d of the thermal change layers.
More particularly, in the conventional technique employing the xe2x80x9cmagnetic transition temperaturexe2x80x9d of the Cr film, since this technique is based on the principle that the specific heat changes only around the magnetic transition temperature, the thermal conductivity of the thermal change layer is large at temperatures sandwiching and near a changing temperature of the thermal change layer. Therefore, there arise problems in that at recording, the shape of bits deforms and at reproduction, the shape of an aperture in the magnetic superresolution deforms.
In the technique using the TiN film, ZrN film and SnO2 film, the thermal conductivity changes smoothly and slightly. That is, it is difficult to obtain a sharp change in the temperature distribution in the light spot.
On the other hand, in the technique using the phase change from solid to liquid of the low-melting metal, the thermal conductivity changes sharply. However, this technique utilizes the change of the thermal conductivity caused by melting of the material, which may possibly involve a change in the thickness of the layer and diffusion of the material into other layers. This restricts repeated use of the medium and mars the reliability of the medium. Especially, since high reliability is a feature of magneto-optical memories and thermomagnetic recording media, significant marring of the reliability through repeated use is a serious problem because such marring destroys the feature of the medium itself.
An object of the present invention is to provide a thermomagnetic recording medium, a thermomagnetic recording and reproducing method and a thermomagnetic recording device which are capable of recording and reproducing at an improved density without reducing the light spot diameter, wherein the temperature distribution in the thermomagnetic recording medium is controlled to realize recording on and reproducing from a track narrower than the light spot, with use of a thermal change layer (1) providing a sharp change in temperature and (2) having a high reliability.
For achieving the above-mentioned object, the thermomagnetic recording medium of the present invention is characterized by comprising at least a recording/reproducing layer and a thermal change layer, wherein a thermal conductivity of the thermal change layer is induced to change by a metal-to-insulator transition as a phase transition between solid-to-solid phases.
For accomplishing the present invention, the inventors, taking notice of the fact that the change of the thermal conductivity is represented by the sum of the thermal conductivity of carriers (electrons or holes) and that of a lattice, utilize a kind of phase transition from one solid phase to another solid phase in the thermal change layer. According to the present invention, a sharp change in the thermal conductivity is obtained within a recording and reproduction temperature range, and a high-density recording is achieved by the thermomagnetic recording medium provided with the reliable thermal change layer, the thermomagnetic recording and reproducing method and the thermomagnetic recording and reproducing device.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.